Steel members such as gears used in automobile transmissions are required to be high in pitting resistance and bending fatigue strength, and to meet such requirement, increasing strength by a carburizing process or a nitriding process has been in practice as a method to strengthen steel members such as gears.
It has been conventionally known that, for improving pitting resistance and bending fatigue strength of a steel member, it is effective to generate an iron nitride compound layer whose main component is a γ′ phase, on a surface of the steel member by a nitriding process, as described in, for example, Patent Document 1.
Further, as a nitriding process method capable of making nitrogen contained in a steel member uniformly from its surface layer up to deep portion in a short time, Patent Document 2 describes that, after a nitriding process is performed in, for example, a 100% NH3 atmosphere in a heating furnace, a nitriding process is performed under a lower NH3 gas concentration than the above, for example, 50% and a 50% N2 gas concentration.